This study utilizes stable isotope methodologies to quantify the changes in rates of glucose utilization and amino acid oxidation during fasting in children with hypoglycemic disorders. These methods have been applied to evaluate the mechanisms of hypoglycemia in ketotic and hypoketotic forms of hypoglycemia. Preliminary data in children with genetic defects in fatty acid oxidation suggest that they do not have excessive glucose utilization rates, but that, as in normal children, hypoglycemia occurs as a consequence of limited capacity for glucose production. Analysis of changes in amino acid kinetics during fasting may reveal whether this limitation reflects a mismatch between rates of glucose utilization and rates of substrates for glucose production.